The essential in 30 seconds
- Through-hull = maximum accuracy. Depth sounder recessed into the hull, in direct contact with water. Depth accuracy ±0.1 m, speed ±0.1 knot, temperature ±0.5°C. Installation only in dry dock: hull drilling, 4-6 hours labor.
- Transom-mount = simplicity. Secured by clamp on the transom, partially submerged. Installation in 1-2 hours without water exit. Loss of accuracy: 10-15% on speed depending on point of sail (cavitation at high speed), depth OK up to 50 m.
- Offshore racing = through-hull by default. All performance sailboats (IMOCA, Class40, Mini) use through-hull depth sounders — accuracy and stability essential for VMG calculation and polars.
- Coastal cruising = transom-mount is enough. For 80% of 30-40 ft cruising sailboats, transom-mount is largely sufficient. Installation cost saving 200-400 €, ease of maintenance (removable at the dock).
- Brands distributed: Airmar (worldwide OEM standard), Raymarine (native Axiom integration), B&G (Triton²/H5000 integration). 2026 HT prices: through-hull depth sounder 250-450 €, transom-mount 80-180 €.
The depth sounder is the #1 sensor for coastal navigation. It measures depth, surface speed, and water temperature — three critical data points for safety (grounding), performance (VMG, polars), and strategy (thermal fronts, currents). The choice between through-hull and transom-mount impacts 10-15 years of sailing.
This article compares the two families technically, presents the brands distributed by Skysat, and provides a workshop verdict by program. For the GPS-depth sounder combo that uses these measurements, see our GPS-depth sounder 9 vs 12 inches for sailboats 2026.
How a marine depth sounder works
A depth sounder combines three distinct sensors in one housing:
- Ultrasonic transducer for depth. Emits a sound pulse (typically 50 or 200 kHz) and measures the round-trip time to the bottom.
- Paddlewheel or electromagnetic sensor for surface speed. The paddlewheel turns with the water flow; the electronics count the rotations.
- Thermistor for water temperature. Accuracy ±0.5°C.
Modern 2026 depth sounders often add a fourth sensor: downvision/sidevision (advanced sonar imaging for fishing or obstacle detection). On a cruising sailboat, this is unnecessary — useful for fishing or navigating in uncharted areas.
Through-hull depth sounder — technical details
The through-hull depth sounder (through-hull in English) is recessed into the hull, in direct contact with water. It replaces a portion of the hull with a bronze cylinder (older models) or high-tech plastic (modern generation).
- Installation: exclusively in dry dock. Hull drilling, epoxy or Sika sealing, validated by controlled water testing. 4-6 hours of workshop labor.
- Optimal position: forward of the boat, under the keel, away from bow waves and propeller bubbles. On a 40 ft sailboat, typically at 1/3 of the length from the bow.
- Accuracy: ±0.1 m on depth up to 200 m, ±0.1 knot on speed at all points of sail (no cavitation), ±0.5°C on temperature.
- Maintenance: annual cleaning of barnacles, paddlewheel replacement every 5-7 years (~ 80 € per unit). The housing remains in place for 15-20 years.
Transom-mount depth sounder — technical details
The transom-mount depth sounder is secured by a clamp on the transom, the vertical element outside the stern. It is submerged when the boat is at rest or moving slowly.
- Installation: 1-2 hours, no water exit required. Light drilling of the transom for 4 stainless steel screws, silicone gasket, cable routed through a watertight gland.
- Optimal position: opposite side of the propeller (to avoid propeller cavitation), 10-15 cm below the waterline, aligned with the transom.
- Accuracy: ±0.2 m on depth up to 50 m, ±0.3-0.5 knot on speed depending on point of sail (cavitation beyond 8 knots), ±0.5°C on temperature.
- Maintenance: removable from the dock (4 screws), quick cleaning. Paddlewheel replaceable in place without water exit.
- Limitations: beyond 8-10 knots, cavitation creates bubbles around the paddlewheel that distort the measurement. On a fast sailboat (Pogo, JPK, racer), this is unacceptable.
Airmar — the worldwide OEM standard
Airmar (New Hampshire, USA) supplies depth sounders for Raymarine, B&G, Garmin, Lowrance, Furuno — 90% of the recreational marine market. When you buy a "Raymarine depth sounder," it’s actually an Airmar depth sounder with a Raymarine connector.
Skysat distributes the Airmar range as white-label for multi-brand refits (you keep your Axiom chartplotter but change the depth sounder), as well as co-branded Raymarine/B&G/Garmin depth sounders for native installations.
Technical comparison chart
2026 HT prices for authorized distributor for standard 50/200 kHz + speed + temperature depth sounder, excluding cabling and workshop installation.
| Criteria | Through-hull | Transom-mount |
|---|---|---|
| Depth sounder price HT | 250-450 € | 80-180 € |
| Workshop installation | 4-6 h (dry dock) | 1-2 h (from dock) |
| Installation cost | 200-400 € | 50-100 € |
| Depth accuracy | ±0.1 m up to 200 m | ±0.2 m up to 50 m |
| Speed accuracy | ±0.1 knot at all points of sail | ±0.3-0.5 knot, cavitation > 8 knots |
| Temperature accuracy | ±0.5°C | ±0.5°C |
| Maintenance | Water exit for paddlewheel (5-7 years) | Removable from dock |
| Housing lifespan | 15-20 years | 10-15 years (UV + impacts) |
| Racing compatible | Yes, by default | No (cavitation) |
Workshop verdict by program
Coastal cruising 30-40 ft — transom-mount
Skysat solution: Airmar P79 or Raymarine CPT-S transom-mount depth sounder (~ 130 € HT) + 1-2 h installation. Total ~ 250 € HT. Accuracy more than sufficient for summer cruising, maintenance simplicity gain.
Offshore cruising 40-50 ft — through-hull
Solution: Airmar P319 bronze through-hull or Raymarine CPT-S BR through-hull (~ 380 € HT) + 4-6 h dry dock installation. Total ~ 700 € HT. Accuracy essential for estuaries and true wind compass navigation.
Offshore racing IMOCA / Class40 — dual through-hull
Solution: 2 × performance through-hull depth sounders (primary + forward redundancy) Airmar (~ 450 € HT each). Total ~ 1 500 € HT installed. Without redundancy, depth sounder failure = no polars = no race strategy.
5 installation errors observed
Depth sounder errors — seen in Skysat workshop
- Transom-mount on the propeller side. Propeller cavitation destroys speed measurement from 4-5 knots. Always mount on the opposite side of the propeller.
- Through-hull positioned too far aft. Too close to the keel = keel bubbles during waves. Optimal position = 1/3 forward from the bow, central axis.
- Neglected sealing gasket on through-hull. #1 cause of flooding in recreational sailing. Always use Sikaflex marine or 2-component epoxy, never standard building silicone.
- Cable routed next to engine or winch cable. Depth sounder cable carries a weak signal. Electrical noise = erratic measurements. Always separate signal from power cables.
- No post-installation speed calibration. Surface speed must be calibrated against GPS (ground speed) in calm seas with no current. Without this step, the surface vs ground speed discrepancy is unusable for current calculation.
FAQ — Practical sailboat depth sounders
Can you convert a transom-mount depth sounder to through-hull?
Not directly — they are two different housing types. Conversion = new through-hull depth sounder + dry dock + hull drilling. If you plan to do this, schedule it during the next haul-out: ~700 € HT total (depth sounder + labor).
How long does a paddlewheel last?
Typically 5-7 years in cruising (300-500 hours sailing/year). In intensive racing 1 500 hours/year, 2-3 years. Spare part: 60-90 € HT, replaceable without removing the depth sounder on most modern models.
50 kHz or 200 kHz depth sounder?
50 kHz = depth (up to 1 000 m), wide beam 30-40° (full coverage). 200 kHz = high resolution (detailed relief), narrow beam 10-15° (vertical accuracy). For cruising, dual 50/200 kHz = good versatility.
CHIRP — really useful?
CHIRP (Compressed High Intensity Radar Pulse) = depth sounder that sweeps a range of frequencies instead of emitting a single frequency. Advantage: better resolution, separation of close targets near the bottom (fish + structure). Useful for fishing, less so for pure cruising. Extra cost ~ 200 € HT vs standard depth sounder.
What is the maximum depth for a recreational depth sounder?
200 kHz alone: 100-150 m useful. 50 kHz: 600-1 000 m depending on power. Beyond that, commercial professional depth sounder (Furuno FCV, Simrad ES) at 3 000-8 000 €. For recreational cruising, dual 50/200 kHz covers 99% of situations.
3 real-world workshop cases — experience feedback
Case 1 — Sun Odyssey 410 sailboat (annual cruising)
Winter refit 2024-2025. Client wanted to replace an old Airmar 235M mechanical depth sounder with a modern CHIRP solution. Recommendation: Airmar B175H 1 kW through-hull depth sounder (~ 480 € HT). Dry dock installation 5 h. GPS calibration post-water testing. Depth accuracy ±0.1 m up to 200 m, stable reading up to 9 knots (propulsion cavitation limit). User ROI: better detection of rocky bottoms in the Saint-Malo estuary, enhanced safety.
Case 2 — Beneteau Oceanis 35.1 (summer coastal)
First depth sounder installation 2025. Choice: Raymarine CPT-S BR transom-mount depth sounder (~ 165 € HT) + Axiom+ 9 integration. Installation 1.5 h from the dock, no water exit required. Depth up to 50 m more than sufficient for coastal navigation. Accepted limitation: reliable speed up to 6.5 knots (inaccurate beyond). Total installation cost: 280 € HT. Client feedback: "exactly what I needed for my use."
Case 3 — Class40 IRC race (offshore competition)
Complete refit 2024. Installation: 2 × Airmar P319 bronze through-hull depth sounders (forward + aft) for race redundancy (~ 380 € HT each). Integration with NKE Multigraphic and Gyropilot 3 computer. Depth accuracy ±0.1 m. Surface speed calibrated against GPS over 8 workshop sessions before the Route du Rhum departure. Material cost: 760 € HT + 12 h workshop labor.
Alternatives ruled out — why certain technologies are not recommended
- Interlift depth sounders (retractable): allow raising the depth sounder into the hull at sea. Technically appealing but sealing joints = permanent weak point. Over 8 years, 60% of interlift depth sounders we replace in the workshop have minor leaks. We do not recommend for recreational sailing.
- "Tap-fix" silicone depth sounders: glued against the hull from the inside (no drilling). Strong marketing promise (30 min installation). Reality: accuracy -30% to -40% vs traditional through-hull, and modern fiberglass hulls (balsa sandwich, foam) are incompatible with this technology. Only relevant for certain aluminum or solid wood hulls.
- Premium SideVision / DownVision: advanced sonar imaging for fishing. For cruising sailboats, extra cost ~ 200-400 € HT is unjustified. Relevant for fishing boats, not sailboats.
3 advanced errors — beyond the basics
Subtle errors — observed on theoretically correct installations
- Through-hull depth sounder misaligned with water flow. The depth sounder has a flow direction (arrow molded on the housing). Installed 180° reversed, it still measures depth but speed readings are erratic. Error observed 2-3 times per year in DIY installation rework.
- Depth sounder coaxial cable in the same conduit as the fluxgate compass cable. The compass measures very weak magnetic fields. The depth sounder cable carries a CHIRP signal with several kW peak power (in short pulses). Coupling via induction → compass drift after a few months. Always maintain a minimum 30 cm gap between the two.
- Surface speed calibration never redone after antifouling. New antifouling slightly alters the flow around the paddlewheel. Initial calibration right after water testing = -5 to -10% reading after a few months. Recalibrate mid-season.
Skysat distributes Airmar, Raymarine, B&G, Garmin, and Navico brands. This article reflects our experience installing depth sounders on over 100 sailboats between 2020 and 2026. 2026 HT prices are indicative for authorized distributor, excluding workshop labor.
